The Citadel is the centre of galactic civilization. It is a space station in the Serpent Nebula inhabited and shared by several different intelligent races. Its rulers, the Citadel Council, are the supreme authority in the Milky Way galaxy. It was originally thought to be created by the Protheans, but it is in fact a Reaper creation designed to allow them to return to the galaxy at any point.
Shepard travels to the Citadel from Eden Prime to report Saren's treachery, and there meets Garrus Vakarian and Urdnot Wrex, and earns his Spectre status. He returns to the Citadel at the end of the game to defeat Saren Arterius and Sovereign and decide the fate of the Council.